De Gea best season was 0,34 PSxG/90, that is exactly the level that Vicario is on now, Onana is on 0,25 so close (but not quite there). The problem is De Gea has not been anywhere close to that since 2018, so purely judging by this parameter Onana is doing a good job and he IS an upgrade on Dave - at least in the league. In CL he has been a disaster. I also think Onana is on the "lucky" side but this evens out in the end.
De Gea flattered to deceive as a shotstopper in the last 5 years, although he was capable of stopping balls that Onana will not be able to reach IMO. He was a poor goalkeeper and was correct decision to get rid.